Dynamic Characteristics of Water Droplets on a Hydrophobic Membrane under External Acoustic Excitation

Acoustic excitation of water droplets on hydrophobic surfaces alters the shape of droplets and enhances their mobility while presenting potential benefits for self-cleaning operations, which become a necessity for practical applications. The present study examines the dynamic response of water droplets on a hydrophobic low-density polyethylene membrane subjected to an external acoustic excitation. Experiments were conducted to evaluate the dynamic response of the droplets via a high-speed imaging system for detailed observations. Droplet oscillations were analyzed while incorporating droplet size, surface wetting state, fluid properties, and acoustic excitation parameters. The membrane surface was hydrophobized by applying a coating of functionalized nanosilica particles, which resulted in a contact angle of ∼148° ∓ 2° with a hysteresis of about ∼2° ∓ 1°. The results reveal that substantial shape deformation and droplet break-off can occur at resonant frequencies. The droplet puddle under gravity reduces droplet natural frequency by a factor of formula presented ; here, dm is the distance between the hydrophobic surface and droplet mass center and Rd is the droplet radius, which is more pronounced for larger droplets (60 μL). At resonance frequencies, oscillations alter droplet shape considerably and necking leads to break-off and formation of newborn droplets (e.g., 212.6 Hz resonance frequency for a 40 μL droplet). The break-off dynamics followed a power-law relation: formula presented ; here, Rnec is the necking radius and tbreak-off is the droplet break-off time, and a newborn droplet with radius ∼5 mm occurs at a break-off time of ∼0.0075 s for a 40 μL droplet. The findings elucidate droplet dynamics under acoustic excitations, pertinent to self-cleaning and fluidic applications, and provide insight into the droplet necking and break-off.